Patent Application
20060086557
The present invention relates generally to power steering systems for vehicles, and more particularly to hydro-mechanically coupled electrically powered steering systems.
Currently it is anticipated that an overwhelming majority of vehicular power steering systems will be electrically powered in the future. Most common will be electric power steering systems (hereinafter “EPS systems”) wherein motors directly deliver steering force as a function of current applied to them by a controller. Because such motors directly deliver steering force, all EPS systems must have an absolute failsafe shutdown mode that unfailingly causes the system to revert to manual steering in the event of any sub-system failure. Such shutdown modes must at a minimum deactivate the system motor.
On the other hand, electro-hydraulic power steering systems (hereinafter “EHPS systems”) utilize a surplus of pressurized fluid flowing through a driver controlled open-center four-way valve to control operation of a system power cylinder (e.g., similarly to present engine pump driven hydraulic power steering systems but in this case with the pressurized fluid provided by an electric motor driven pump). Since a driver directly controls steering motion in a vehicle equipped with an EHPS system, they are inherently failsafe in operation. EHPS systems are not considered acceptable for utilization in future vehicles however, because they waste a significant percentage of the pressurized fluid in all but “accident avoidance” maneuvers in order to effect differential output pressure control. Thus they are in general not capable of being utilized for vehicles with larger steering loads, such as for instance, typical medium sized automobiles. Thus as defined herein, an EPS system is one wherein a motor directly delivers steering force whenever providing steering assist, while in an EHPS system steering force is generated by an open-center valve selectively metering a continuous flow of pressurized fluid there through in order to generate differential pressure across a power cylinder.
A particularly desirable hydro-mechanically coupled EPS system is described in U.S. Pat. No. 6,152,254, entitled “Feedback and Servo Control for Electric Power Steering System with Hydraulic Transmission,” issued Nov. 28, 2000 to Edward H. Phillips. Because of continued reference to the '254 patent hereinbelow, the whole of that patent is expressly incorporated in its entirety by reference herein. In that system, differential pressure is directly delivered to a double-acting power cylinder from a motor driven pump, whereby it is differentiated from an EHPS system in accordance with the above definition in that all of the pumped pressurized fluid is directly applied to assisted steering of a host vehicle. In any case, one of that EPS system's most desirable features is that in addition to deactivating the system motor when activating a shutdown mode as called for above, both ports of its system power cylinder are faulted to each other and a system reservoir in the event of any sub-system failure as redundant shutdown mode measure.
In all presently known EPS systems however (e.g., even including the hydro-mechanically coupled EPS system described in the incorporated '254 patent), all shutdown modes are software controlled and electronically implemented whereby there exists a finite possibility that all such shutdown modes could simultaneously fail and that a resulting runaway steering event could occur. This possibility is exacerbated by the fact that apparatus supporting these safety shutdown modes must survive indefinitely through changes of vehicle ownership, indifferent maintenance scenarios, and even tinkering by unqualified individuals.
Further and similarly to all other known EPS systems, the hydro-mechanically coupled EPS system described in the incorporated '254 patent has the undesirable characteristic of reflecting motor inertia back to the vehicle's steering wheel whenever negligible power assist is required such as during on-center operation and/or at very high vehicular speeds. This is especially bothersome because the motor inertia is compliantly coupled to the steering wheel (i.e., via a torsion bar).
A hydro-mechanically coupled EPS system according to the present invention functions in an inherently failsafe manner and avoids reflecting motor inertia back to the vehicle's steering wheel whenever negligible power assist is required.
The hydro-mechanically coupled EPS system (which, again is differentiated from an EHPS system because in accordance with the above definition its motor directly delivers steering force whenever providing steering assist) includes a double-acting power cylinder having left and right cylinder ports and a directional control valve. The directional control valve is preferably an under-lapped four-way valve and has an input port, a return port fluidly connected to a reservoir and left and right output ports respectively fluidly connected to the left and right cylinder ports. Pressurized fluid can be delivered to either of the left and right cylinder ports during transition by the directional control valve through and beyond its underlap region. An electronically controlled motor driven pump has an output port fluidly connected to the input port of the directional control valve.
The hydro-mechanically coupled EPS system further includes a steering wheel torque transducer for providing an applied torque signal Vat indicative of values of torque applied to the steering wheel (hereinafter optionally “applied torque”). An optional pressure transducer provides a pressure signal Vp indicative of pressure values present at the input port of the directional control valve. A controller provides a power control signal Ve to the motor driven pump based upon the difference between a control function signal Vcf determined by a control algorithm from at least the magnitude of the applied torque signal Vat and the pressure signal Vp issued by the pressure transducer. The motor driven pump is controlled such that pressurized fluid is supplied to the input port of the directional control valve at fluid pressure values that continually move toward the control function signal Vcf.
In the case of a system not utilizing the optional pressure transducer, the controller provides a power control signal Vc to the motor driven pump at values of the power control signal Vc determined directly from the control function signal Vcf. Thus in either case, pressurized fluid is provided by the directional control valve to one of the ports of the double-acting power cylinder as determined by the rotational direction of the applied torque at a value in accordance with the magnitude of the applied torque and the resulting control algorithm determined control function signal Vcf.
It is desirable to decouple the motor inertia from the steering wheel during “on-center” steering conditions. This is accomplished by fluidly coupling both of the left and right cylinder ports to the reservoir during on-center operation. This is realized by fluid freely flowing through the orifices of the directional control valve when in a true centered position and progressively otherwise by virtue of either one of the left and right output ports of the directional control valve being predominately fluidly connected to the reservoir via a check valve and its input port, and the other one being predominately fluidly connected to the reservoir by its return port depending upon the direction of the torque applied to the steering wheel. Of course, this mode of operation requires that the directional control valve transition through its underlap region (e.g., the various control orifices there within achieve fully closed status) at relatively low values of applied torque (i.e., ±10 in.lbs.) and that the control algorithm is configured such that the control function constant Kcf has substantially zero values for lesser values of applied torque.
Inherently failsafe operation of the hydro-mechanically coupled EPS system is provided by the directional control valve directly controlling fluid flow to the ports of the double-acting power cylinder in the manner of present engine pump driven hydraulic power steering systems. Directional control of pressurized fluid applied to the double-acting power cylinder is manually implemented by driver control of the directional control under-lapped four-way valve via the steering wheel and a steering shaft connected thereto.
A method for enabling a hydro-mechanically coupled power steering system includes the step of fluidly connecting the input port of the pump to the reservoir and fluidly connecting the output port of the pump to the input port of the directional control valve. The pressure transducer is connected to the input port of the directional control valve. The return port of the directional control valve is connected to the reservoir. Torque applied to the steering wheel is measured and a signal representative of the magnitude of the applied torque is provided. A signal representative of a desired pressure value to be applied to the input port of the directional control valve is determined as a selected function of at least the magnitude of the applied torque. The pressure value actually present at the input port of the directional control valve is measured and subtracted from the desired instant pressure value to form an error signal. The error signal is filtered and amplified to form a power control signal and the pump is operated in response to the power control signal so as to continually reduce the error signal and thus provide the desired pressure value to the input port of the directional control valve.
Because of its inherently failsafe operational characteristics under all operating conditions, and further because of its improved steering feel whenever negligible power assist is required such as during on-center operation or at very high vehicular speeds, the hydro-mechanically coupled EPS systems of the present invention posses distinct advantages over all known prior art EPS systems.
Other advantages of the present invention can be understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
With reference first to
Operationally, whenever torque is applied to the steering wheel 12, and particularly whenever that applied torque is sufficient to effect translation through and beyond the underlap region of the directional control valve 20, an applied torque signal Vat is sent to the controller 30 by a torque transducer 32 operatively connected thereto via steering shaft 84. Then the absolute value of the applied torque signal Vat is multiplied by a control function constant Kcf to form a control function signal Vcf, where the control function constant Kcf is generated by the controller 30 as a function of at least the applied torque value, and most probably vehicular speed, in accordance with procedures fully explained in the incorporated '254 patent. A pressure signal Vp from a pressure transducer 34 provided for measuring pressure values in the fluid line 18 is then subtracted from the control function signal Vcf whereby the resulting algebraic sum forms an error signal Ve. The error signal Ve is then filtered and amplified to form a power control signal Vc that is then continuously applied to the motor 28 in such a manner as to cause the error signal Ve to decrease in value.
Economically viable candidate pressure transducers (e.g., practical pressure transducers for utilization as the pressure transducer 34) are generally subject to temperature drift. Thus it is desirable to provide a method for automatically calibrating the pressure transducer 34 immediately before and during operation of the inherently failsafe hydro-mechanically coupled EPS system 10, wherein in this case calibration is defined as resetting the pressure signal Vp to a zero value whenever it can be assured that fluid pressure in the fluid line 18 has a value equal to reservoir pressure. Obviously, this is true immediately before operation of the host vehicle. Thus, the turn-on sequence for the inherently failsafe hydro-mechanically coupled EPS system 10 comprises first “waking up” the controller 30, then calibrating the pressure signal Vp to a zero value, and finally operationally activating the inherently failsafe hydro-mechanically coupled EPS system 10. During operation, calibration to a zero value is accomplished whenever the pressure signal Vp value achieves a minimum value as defined by its time-based differential concomitantly achieving a zero value.
In any case, the inherently failsafe hydro-mechanically coupled EPS system 10 of the present invention (e.g., along with the EPS system with hydraulic transmission (10) described in detail in the incorporated '254 patent) provides steering accuracy and stability unmatched by any other known power steering system because its operation is controlled in an internal feedback loop in accordance with the above described method of causing the error signal Ve to continually decrease in value. This is especially so when performance of the inherently failsafe hydro-mechanically coupled EPS system 10 is compared to that of typical mechanically coupled EOS systems. This especially favorable comparison is due to Coulomb friction that is typical in their drive gear trains.
With reference now to
With additional reference now to
The directional control valve 20 is formed in an underlapped manner as a consequence of the input and return slots 56 and 58, and left and right output slots 60a and 60b all being formed with greater circumferential widths than juxtaposed lands 68 whereby input orifices 70a and 70b, and return orifices 72a and 72b are all enabled for freely conveying fluid in the on-center position as illustrated in
It is desirable for working pressures in the double-acting power cylinder 16 to always be kept at the lowest pressure values possible. This keeps pressure values applied to various power cylinder seals to a minimum thereby reducing leakage problems and minimizing Coulomb friction. The directional control valve 20 automatically accomplishes this task of course because at least one set of the left and right output slots 60a and 60b is always fluidly connected to the return slots 58 and thus the reservoir 24.
In addition, it is also desirable to fluidly couple both of the left and right output slots 60a and 60b (and thus the left and right cylinder ports 14a and 14b) to the reservoir 24 during “on-center” steering conditions. This precludes a possible problem wherein foam could form in the fluid due to rapid cycling of the steering wheel 12. This problem could arise due to pressure drop within either side of the double-acting power cylinder 16 relative to reservoir pressure. Such pressure drop could result from backflow through a respective one of the return orifices 72a and 72b of the directional control valve 20 when rapidly recovering from a turn. A practical solution is to provide a check valve 74 fluidly connected between the reservoir 24 and the fluid line 18 as shown in both of
In the event of any system failure, a primary failsafe shutdown procedure is implemented via the controller 30 precluding current from being applied to the motor 28 whereby manual steering is imposed regardless of steering load. Thus for applied torque values resulting in transition through and beyond the directional control valve 20's underlap region, fluid would enter fluid line 18 through the check valve 74 and then flow through the directional control valve 20 and power cylinder 16 in the manner already described. In addition however, a redundant failsafe feature is provided via the directional control valve 20 directly controlling fluid flow to the ports 14a and 14b of the double-acting power cylinder 16 in the manner of present engine pump driven hydraulic power steering systems wherein directional control of pressurized fluid applied to the double-acting power cylinder 16 is manually implemented by driver control of the directional control valve 20 via the steering wheel 12 and the steering shaft 84.
Admittedly the resulting steering feel would be rather “light” during such an emergency event. For instance, starting with the above example where the on-center circumferential width of the orifices 70a, 70b, 72a and 72b is 0.010 in. along with an orifice flow equation presented in a book by Herbert E. Merritt entitled “Hydraulic Control Systems” and published by John Wiley & Sons, Inc. of New York, the amount of circumferential closure of either set of orifices 70a and 72b or 70b and 72a required to effect a nominal differential power cylinder pressure of say 100 lbs./in.2 is determined by the following equation:
x=0.010−(Q/2)/(70 w Sqrt[deltaP])=0.009 in.
wherein the following assumptions have been made: x is the amount of circumferential closure, Q is the pump flow rate=6 Liters/min., w is the combined axial length of any of the orifices 70a, 70b, 72a or 72b=4 in., and deltaP is pressure drop through either of the closing orifices=100 lbs./in.2. Then again following above example, the resulting applied torque value is 9 in.lbs. The resulting steering feel would indeed be “light,” but it is certainly preferable to experiencing a runaway steering event that is theoretically possible with other known EPS system wherein a motor is directly linked to the dirigible wheels and thus that EPS system is absolutely dependent upon software controlled and electronically enabled failsafe apparatus and procedures for an orderly absolute failsafe shutdown. Thus both of the inherently failsafe hydro-mechanically coupled EPS systems 10 and 40 can indeed uniquely be said to be inherently failsafe EPS systems.
As depicted in the flow chart of
Finally, as depicted in the flow chart of
Having described the invention, however, many modifications thereto will become immediately apparent to those skilled in the art to which it pertains, without deviation from the spirit of the invention. For instance, it would be possible to configure either of the hydro-mechanically coupled EPS systems without the check valve 74. Or an additional failsafe feature could be provided in the form of a normally open, solenoid-controlled two-way valve fluidly connecting the fluid line 18 to the reservoir 24 unless its solenoid was activated. Such modifications clearly fall within the scope of the invention.
The instant systems are capable of providing inherently failsafe and otherwise improved power steering systems intended for small through medium sized vehicles, and accordingly find industrial application both in America and abroad in power steering systems intended for such vehicles and other devices requiring powered assist in response to torque applied to a steering wheel, or indeed, any control element functionally similar in nature to a steering wheel.
This application claims priority to U.S. Provisional Application Ser. No. 60/621,797 filed Oct. 25, 2004.
| Number | Date | Country | |
|---|---|---|---|
| 60621797 | Oct 2004 | US |
